Sesamoids and accessory ossicles of the foot vary widely in their prevalence and morphology.
There are almost 40 acessory ossicles described and the more common ones are: os peroneum; os subfibulare; os subtibiale; os tibiale externum; os trigonum; os calcaneus secundaris; os intermetatarseum; os supratalare; os supranaviculare and bipartite hallux sesamoid.
Sesamoid bones are generally thought to form from a combination of biological and mechanical factors,
beginning as cartilaginous nodules that then undergo endochondral ossification during early to late childhood.
Accessory bones are supernumerary bones that commonly derive from unfused primary or secondary ossification centers and don’t have a well-known function.
Fig. 1: Acessory bones of the foot - AP (a), oblique (b) and lateral (c) - 1 Os trigonum, 2 os peroneum, 3 os naviculare, 4 os intermetatarseum, 5 os vesalianum 6 os supranaviculare, 7 os supratalare, 8 os talotibiale, 9 os calcaneus secundarium
References: Insights Imaging (2013) 4:581–593
Table 1: Accessory and sesamoid bones: locations and prevalence